1. Field of the Invention
The present invention relates to two-component aqueous polyurethane dispersions which may be cured at ambient temperature and to the coatings prepared therefrom which have excellent hardness, flexibility, solvent resistance and surface appearance.
2. Description of the Prior Art
Aqueous polyurethane dispersions and their use for the production of coatings is known. The dispersions may be cured at ambient temperature by evaporation of water and coalescence of the individual polyurethane particles. These aqueous-based products have been developed in an effort to reduce the amount of organic solvents which are present in corresponding solvent-based coating compositions. Even though the prior art dispersions possess many valuable properties, it has not been possible to obtain coatings which possess all of the properties of coatings obtained from solvent-based coating compositions, especially hardness and solvent resistance.
The known aqueous polyurethane dispersions do not possess the amount of crosslinking which is required to obtain these properties. One method of increasing the amount of crosslinking is to blend the polyurethane dispersion with a water dispersible, blocked polyisocyanate as disclosed in U.S. Pat. No. 4,098,933. U.S. Pat. No. 4,608,413 discloses the use of water dispersible, blocked polyisocyanates in combination with polyurethanes which contain hydroxyl groups as crosslinking sites. While the systems disclosed in these patents make it possible to obtain improved hardness and crosslinking, they suffer from the disadvantage that the coating compositions must be heated to high temperatures in order to release the blocking agent and initiate crosslinking. Obviously, these coating compositions are not suitable for application to substrates which cannot withstand the unblocking temperature of the polyisocyanate.
Accordingly, it is an object of the present invention to provide aqueous polyurethane dispersions which may be cured at ambient temperature to provide coatings with excellent hardness, flexibility, solvent resistance and surface appearance.
This object may be achieved in accordance with the present invention as set forth hereinafter by the use of two-component coating compositions wherein one component is an aqueously dispersed, hydroxy functional polyurethane and the second component is a water dispersible, unblocked polyisocyanate.
Previously, it was known from U.S. Pat. No. 4,663,377 to modify aqueous polyurethane adhesive dispersions by the addition of water dispersible, unblocked polyisocyanates. German Offenlegungsschrift 3,728,140 teaches that an improvement in the heat activation temperature of the adhesive dispersions may be obtained by chain extending the polyurethane with a monoamine containing at least one hydroxyl group. Neither of these references suggest the use of the aqueous dispersions as coating compositions. In fact, it is surprising that the two-component dispersions according to the present invention can be used to prepare acceptable coatings. It would be expected that a portion of the isocyanate groups of the water dispersible polyisocyanate would react with the water present in the aqueous dispersions instead of the hydroxyl groups of the polyurethane because the reactivity of isocyanate groups with water and hydroxyl groups is similar. Because the isocyanate/water reaction produces carbon dioxide as a by-product, it would also be expected that some of the carbon dioxide would be trapped in the form of bubbles within the resulting coating rendering it unacceptable for further use. However, it has surprisingly been found that it is possible to obtain coatings in accordance with the present invention which are comparable in surface appearance to polyurethane coatings obtained from a two-component solvent based coating composition.